The invention disclosed herein pertains to current sensitive switch apparatus and more particularly to overcurrent protective switch apparatus. Still more particularly this invention relates to overload relays and the like which are thermally responsive to increased current levels to interrupt power to a protected device. The invention is particularly adapted for use in protecting electric motors and other load devices against overload currents.
Conventional overload relays utilize current sensing heater coils to actuate spring-loaded latch trip contact actuator mechanisms. Such mechanisms are large and comprise many parts, are difficult and time consuming to assemble and calibrate, and in many embodiments actuate the contacts near the end of travel of the springs when the forces have weakened and are less predictable.
Shape memory effect (SME) alloys may be advantageously employed in overcurrent protective switch apparatus to provide simpler but more efficient switch mechanisms than the aforedescribed types. Elements made of SME alloys are formed to a first configuration, heated and subsequently cooled while restrained in that configuration, and thereafter reformed to a second configuration. The material possesses a transition temperature level and upon being heated to that level, the element reverts back to the first configuration, releasing significant forces in that transformation. By suitably shaping and biasing the element it can be made to return to the second configuration upon cooling below the transition temperature. Biasing the element also permits the temperature range at which the physical transformation occurs to be more precisely controlled. By utilizing SME elements in overcurrent responsive switches of the aforedescribed type, larger and more predictable switch operating forces can be obtained which enables less refined switching contact structures to be employed without concern for trip point variations caused when contact forces offset operating mechanism forces. The SME elements also provide larger amounts of displacement for given temperature increases than are provided by the commonly used bimetal elements, thereby reducing the manufacturing tolerances for the switch apparatus. The SME element is electrically conductive, and thus it may be utilized to directly sense the current level being received by the load device, being heated by current flowing through it to therefor also serve as the heater unit. Thus a single SME element can perform the functions of several distinct elements of a conventional overload relay, and operates in a more positive, improved manner.